'\" et
.TH MQ_NOTIFY "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
mq_notify
\(em notify process that a message is available
(\fBREALTIME\fP)
.SH SYNOPSIS
.LP
.nf
#include <mqueue.h>
.P
int mq_notify(mqd_t \fImqdes\fP, const struct sigevent *\fInotification\fP);
.fi
.SH DESCRIPTION
If the argument
.IR notification
is not NULL, this function shall register the calling process to be
notified of message arrival at an empty message queue associated with
the specified message queue descriptor,
.IR mqdes .
The notification specified by the
.IR notification
argument shall be sent to the process when the message queue transitions
from empty to non-empty. At any time, only one process may be
registered for notification by a message queue. If the calling process
or any other process has already registered for notification of message
arrival at the specified message queue, subsequent attempts to register
for that message queue shall fail.
.P
If
.IR notification
is NULL and the process is currently registered for notification by the
specified message queue, the existing registration shall be removed.
.P
When the notification is sent to the registered process, its
registration shall be removed. The message queue shall then be available
for registration.
.P
If a process has registered for notification of message arrival at a
message queue and some thread is blocked in
\fImq_receive\fR()
or
\fImq_timedreceive\fR()
waiting to receive a message when a message arrives at the queue, the
arriving message shall satisfy the appropriate
\fImq_receive\fR()
or
\fImq_timedreceive\fR(),
respectively. The resulting behavior is as if the message queue remains
empty, and no notification shall be sent.
.SH "RETURN VALUE"
Upon successful completion, the
\fImq_notify\fR()
function shall return a value of zero; otherwise, the function shall
return a value of \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fImq_notify\fR()
function shall fail if:
.TP
.BR EBADF
The
.IR mqdes
argument is not a valid message queue descriptor.
.TP
.BR EBUSY
A process is already registered for notification by the message queue.
.P
The
\fImq_notify\fR()
function may fail if:
.TP
.BR EINVAL
The
.IR notification
argument is NULL and the process is currently not registered.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
The following program registers a notification request for the message
queue named in its command-line argument. Notification is performed
by creating a thread. The thread executes a function which reads one
message from the queue and then terminates the process.
.sp
.RS 4
.nf

#include <pthread.h>
#include <mqueue.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
.P
static void                     /* Thread start function */
tfunc(union sigval sv)
{
    struct mq_attr attr;
    ssize_t nr;
    void *buf;
    mqd_t mqdes = *((mqd_t *) sv.sival_ptr);
.P
    /* Determine maximum msg size; allocate buffer to receive msg */
.P
    if (mq_getattr(mqdes, &attr) == -1) {
        perror("mq_getattr");
        exit(EXIT_FAILURE);
    }
    buf = malloc(attr.mq_msgsize);
.P
    if (buf == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }
.P
    nr = mq_receive(mqdes, buf, attr.mq_msgsize, NULL);
    if (nr == -1) {
        perror("mq_receive");
        exit(EXIT_FAILURE);
    }
.P
    printf("Read %ld bytes from message queue\en", (long) nr);
    free(buf);
    exit(EXIT_SUCCESS);         /* Terminate the process */
}
.P
int
main(int argc, char *argv[])
{
    mqd_t mqdes;
    struct sigevent not;
.P
    assert(argc == 2);
.P
    mqdes = mq_open(argv[1], O_RDONLY);
    if (mqdes == (mqd_t) -1) {
        perror("mq_open");
        exit(EXIT_FAILURE);
    }
.P
    not.sigev_notify = SIGEV_THREAD;
    not.sigev_notify_function = tfunc;
    not.sigev_notify_attributes = NULL;
    not.sigev_value.sival_ptr = &mqdes;   /* Arg. to thread func. */
    if (mq_notify(mqdes, &not) == -1) {
        perror("mq_notify");
        exit(EXIT_FAILURE);
    }
.P
    pause();    /* Process will be terminated by thread function */
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgrcv\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<mqueue.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
